Crystalline aluminate compositions conforming generally to the empirical formula Li.sup.+ (RCOO.sup.-).2Al(OH).sub.3.nH.sub.2 O, where RCOO.sup.- represents an organic acid anion, and nH.sub.2 O represents any waters of hydration, are disclosed, inter alia, in U.S. Pat. No. 4,348,295, U.S Pat. No. 4,348,296, and U.S. Pat. No. 4,348,297. These three patents are incorporated herein by reference.
The following patents are believed to be representative of the most relevant art regarding extreme pressure lubricant additives: U.S. Pat. No. 2,621,159; U.S. Pat. No. 3,001,939; U.S. Pat. No. 3,093,584; U.S. Pat. No. 3,318,808; U.S. Pat. No. 3,565,802; U.S. Pat. No. 3,909,426; U.S. Pat. No. 3,984,599; U.S. Pat. No. 3,997,454; and U.S. Pat. No. 4,293,430.
In these relevant arts the principals of the following tests, or variations thereof, are usually followed:
ASTM D-2509 "Standard Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method)". PA0 ASTM D-2782 "Standard Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Timken Method)". PA0 ASTM D2783 "Standard Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Four-Ball Method)".
In this present disclosure the expression "lithium stearate aluminate" (a.k.a. "LSA") is a crystalline compound of the above formula, Li.sup.+ (RCOO.sup.-).2Al(OH).sub.3.nH.sub.2 O, where RCOO.sup.- is the negative-valent carboxylate radical of stearic acid. It is prepared in accordance with the procedure disclosed in U.S. Pat. No. 4,348,295 or U.S. Pat. No. 4,348,297 whereby crystalline LiOH.2Al(OH).sub.3.nH.sub.2 O, material is reacted with stearic acid, thereby replacing the OH.sup.- (attached to the Li) with RCOO.sup.-.
Lithium stearate aluminates (LSA) are organic-inorganic (60:40) hybrid crystalline materials conforming substantally to the empirical formula LiX.2Al(OH).sub.3.nH.sub.2 O, where X is an anion (stearate) and nH.sub.2 O represents water of hydration. These are 2- or 3-layer unit cell structures. The particle size is usually from about 150 .ANG. to about 5000 .ANG.. TGA studies have shown that it decomposes at 300.degree. C. X-ray defraction and SEM analyses have revealed its platelet structure.
Industrial oils and lubricating fluids frequently require friction reducers for energy saving and antiwear/extreme pressure additives to extend their functional range. Tribological research has now been conducted to evaluate the lubtication performance of lithium stearate aluminate as an additive in lubricants. The place of LSA within the lubrication as an extreme pressure (EP), anti-wear and friction-reducing additive.
Extreme pressure (EP) and anti-wear additives are used mainly to improve the performance of lubricants. As a class, such substances produce a physical or chemical effect on the surfaces of the friction pair, thus leading to a reduction in wear rate under conditions of mixed or boundary lubrication and an increase in the seizure load. Such additives are called extreme pressure and antiwear additives.
The principal effect of an EP additive occurs under heavy loads when, in addition to high temperatures, the metallic surface is activated mechanically (tribochemical effect). It is known that freshly worn surfaces are a source of electrons which are capable of initiating several reactions which would otherwise not occur. In some cases, the additive or additives present in the lubricant, in contact with the frictional surface at high temperatures, undergo polymerization or reaction with one another leading to the formation of a solid compound on the surface. The polymer layer formed by the insitu polymerization at high temperature, affords protection of the metallic surfaces against corrosion, serving as an antioxidant.
It is well known that some extreme pressure additives such as chlorinated paraffins, sulfo-chlorinated oils, or zinc dithiophosphates react with metallic surfaces during the frictional process. The reaction layer may improve the frictional properties of the metallic surfces if it is a low shear strength compound or may simply prevent direct contact between the surfaces and the formation of junctions. Reaction between the metallic surface and the additive may also reduce adhesion. At the point of contact between the surfaces where the temperature is high, the additive prevents the formation of an adhesion bridge by reacting with the metallic surface. As asperities are the initial contact points, the process may lead to polishing of the surface (chemical polishing). Some of the EP additives (e.g. Zn-dithiophosphates) also possess antioxidant characteristics having two or more active elements in their molecules.
If the effectiveness of EP and antiwear additives is due to the reaction layer formed on the metallic surfaces in contact, additive reactivity should be controlled, that is, the reaction between the metallic surface and the additive should take place only on the friction surface. Excessive reactivity may cause corrosion while low reactivity may not permit the formation and preservation of a protective layer on the surfaces of the friction pair as the existence of the layer in the contact area is the result of an equilibrium between the formation and wear processes. For this reason, chlorinated paraffin oils have limited use as antiwear-extreme pressure additive. It is reported that the presence of chlorinated-type additives in metal-working fluids often initiate corrosion of the machines over a long period of idleness. Some chlorine and sulfur base additives are reported to irritate skin or produce foul odors.
Long chain lubes such as the esters of fatty acids, aliphatic alcohols, and amines are used as friction reducers and antiwear additives in lubricants. A characteristic or their effectiveness is determined by the stability of the layer on the frictional surface. Usually at relatively low temperatures up to 150.degree. C., the layer is desorbed, losing its effectiveness. The melting points of most of the long chain lubes (metallic soaps) are less than 150.degree. C. For effective lubrication above 150.degree. C., surface films withstanding higher temperatures must be used. Some lamellar solids, such as graphite and molybdenum disulfide, having low intrinsic shear strength because of their layer lattice crystal structure, are used as solid lubricants. The lubrication effectiveness is attributed to the formation of an adhering film rather than a reactive film. Solid lubricants are well known as friction reducers. Crystalline lithium aluminates (stearic anion) decompose at 300.degree. C., whereas MoS.sub.2 and graphite melt at 400.degree. C. and 500.degree. C.
The particle size usually affects the lubricating properties of the suspension. Experimental work carried out on a four-ball tester has shown that if colloidal suspensions are used, the optimum mean diameter of the MoS.sub.2 particle is around 25,000 .ANG.. A complex interdependence exists between particle size and the antiwear characteristics of the suspension. Under light loads particle size has no effect, while under heavy loads larger grain sizes usually result in increased wear.